1. Field of the Invention
The present invention relates to an ophthalmic apparatus, a method of controlling the ophthalmic apparatus, and a storage medium and, more particularly, to a technique for improving the accuracy of fundus tracking.
2. Description of the Related Art
Currently, an optical coherence tomography apparatus based on OCT (Optical Coherence Tomography) using multiwavelength light wave coherence is known. For example, this apparatus is used in an endoscope to obtain organ information and used in an ophthalmic apparatus to obtain retina information, thus expanding the field of application to the human body. The optical coherence tomography apparatus applied to the eyes is therefore becoming indispensable to outpatient clinics specialized in retinas as an ophthalmic apparatus.
Such an optical coherence tomography apparatus irradiates a sample with measurement light which is low-coherent light, and can measure backscattered light from the sample by using an interference system. This apparatus can obtain image information in the depth direction at a given point on the sample by irradiating the point on the sample with measurement light. The apparatus can also obtain a tomographic image of the sample by performing measurement while scanning measurement light on the sample. When being applied to the fundus, the apparatus can capture a tomographic image of the fundus of the eye to be examined at high resolution by scanning measurement light on the fundus of the eye to be examined, and hence has been widely used for ophthalmic care and the like for the retina.
Optical coherence tomography apparatuses generally use an imaging method of obtaining a plurality of tomographic images by iteratively scanning the fundus as a measurement target, in the horizontal or vertical direction. For example, an optical coherence tomography apparatus can obtain one fundus tomographic image with high image quality by acquiring a plurality of fundus tomographic images of the same region by scanning the same place on the fundus a plurality of times and averaging them. This apparatus can also obtain a three-dimensional image of the fundus by scanning the fundus a plurality of times while translating the scan position. When, however, performing such scanning a plurality of times, the apparatus requires a certain period of time to complete the entire imaging operation. For this reason, the eye may move during the scanning.
In contrast to this, the ophthalmic imaging apparatus disclosed in Japanese Patent Laid-Open No. 2008-29467 uses a method (to be referred to as “fundus tracking” hereinafter) of sequentially capturing front images of the eye to be examined, detecting the movement of the eye by using a plurality of obtained front images, and correcting the scan position in accordance with the detected movement of the eye. As described above, the optical coherence tomography apparatus places importance on the processing of reducing the influence of the movement of the eye to be examined.
In order to obtain a fundus tomographic image with high accuracy at high speed, the optical coherence tomography apparatus disclosed in Japanese Patent Laid-open No. 2012-30054 generates a reference image (to be referred to as a “template” hereinafter) by selecting a fundus observation image serving as a reference from which a feature point as an index of the movement of the eye to be examined has been extracted, generates another template using a fundus observation image of a fundus tracking target which is obtained after the generation of the template, and performs fundus tracking by template matching using the same feature point.
Assume that the eye to be examined has moved during capturing of an image serving as a fundus tracking target. In this case, vignetting or the like at the pupil of the eye may cause illuminance unevenness on the image. At this time, the contrast decreases at a feature point, in particular, at a characteristic region such as a blood vessel region, in the template of the image as the fundus tracking target. This may cause errors in the detection accuracy of template matching. As a result, an error in the scan position due to fundus tracking will cause displacement (distortion) on an acquired tomographic image. This distortion on the tomographic image may hinder image diagnosis by the doctor and also may make him/her mistake the distortion on the tomographic image for a morbid, portion. This may lead to misdiagnosis.